Isoolefin polymers, and in particular, hydrocarbon-rubbers, may be prepared in carbocationic polymerization processes. See, e.g., Organic Chemistry, SIXTH EDITION, Morrison and Boyd, Prentice-Hall, 1084-1085, Englewood Cliffs, New Jersey 1992, and K. Matyjaszewski, ed, Cationic Polymerizations, Marcel Dekker, Inc., New York, 1996. The catalyst system for producing hydrocarbon-rubbers typically includes two components: an initiator and a Lewis acid. Examples of Lewis acids include AlCl3 and BF3. Examples of initiators include Brønsted acids such as HCl, RCOOH (wherein R is an alkyl group), and H2O. During the polymerization process, in what is generally referred to as the initiation step, the isoolefin, e.g., isobutylene, reacts with the Lewis acid/initiator pair to produce a carbenium ion. Following the initiation step, additional monomer units add to the formed carbenium ion in what is generally referred to as the propagation step. These steps typically take place in a diluent or in a solvent.
Industry has generally accepted widespread use of a slurry polymerization process to produce hydrocarbon-rubbers, using methyl chloride (MeCl) as the diluent. Typically, the diluent used in slurry polymerization processes consists essentially of methyl chloride. Methyl chloride is employed for a variety of reasons, including the ability of methyl chloride to dissolve the monomers and the aluminum chloride catalyst of the reaction mixture, but not dissolve the hydrocarbon-rubber polymer product of the polymerization process. Methyl chloride also has a suitable freezing point to permit low temperature polymerization, typically at temperatures less than or equal to −90° C. Methyl chloride also has a suitably low boiling point to allow for effective separation of the hydrocarbon-rubber polymer from the diluent. A slurry polymerization process using methyl chloride as the diluent also offers the advantage of a hydrocarbon-rubber polymer concentration of approximately 26% to 37% by volume in the reaction mixture, as opposed to a concentration of only about 8% to 12% in a solution polymerization process, wherein the hydrocarbon-rubber polymer is at least partially dissolved in a solvent. Reaction mixtures using methyl chloride as a diluent also have a relatively low viscosity, enabling the heat of polymerization formed during the polymerization reaction to be removed effectively by surface heat exchange.
Typical commercial reactors used to produce hydrocarbon-rubber in a slurry polymerization process include well mixed vessels with a volume of about 10 to 30 liters, wherein the circulation of the reaction mixture is often provided by a pump impeller. An example of such a reactor includes a continuous flow stirred tank reactor (“CFSTR”) as described in U.S. Pat. No. 5,417,930, which is incorporated by reference herein. For purposes herein, a reactor suitable for use in a slurry polymerization process to produce rubber is referred to in general as a “reactor” or as a “butyl reactor”. In these reactors, slurry is circulated through tubes of a heat exchanger by a pump, while boiling ethylene on the shell side provides cooling, the slurry temperature being determined by the boiling ethylene temperature, the required heat flux and the overall resistance to heat transfer. On the slurry side, the heat exchanger surfaces progressively accumulate polymer, inhibiting heat transfer, which would tend to cause the slurry temperature to rise. This often limits the practical slurry concentration that can be used in most reactors from 26 to 37 volume % relative to the total volume of the slurry, diluent, and unreacted monomers. The subject of polymer accumulation has been addressed in several patents (such as U.S. Pat. No. 2,534,698, U.S. Pat. No. 2,548,415, and U.S. Pat. No. 2,644,809). However, these patents have unsatisfactorily addressed the myriad of problems associated with polymer particle agglomeration for implementing a desired commercial process. Additionally, Thaler, W. A., Buckley, Sr., D. J., High Molecular-Weight, High Unsaturation Copolymers of Isobutylene and Conjugated Dienes, 49(4) Rubber Chemical Technology, 960 (1976), discloses, inter alia, the cationic slurry polymerization of copolymers of isobutylene with isoprene (butyl rubber) and with cyclopentadiene in heptane.
However, there are a number of problems associated with the polymerization in methyl chloride, for example, the tendency of the polymer particles in the reactor to agglomerate with each other and to collect on the reactor wall, heat transfer surfaces, impeller(s), and the agitator(s)/pump(s). The rate of agglomeration increases rapidly as reaction temperature rises. Agglomerated particles tend to adhere to and grow and plate-out on all surfaces they contact, such as reactor discharge lines, as well as any heat transfer equipment being used to remove the exothermic heat of polymerization, which is critical since low temperature reaction conditions must be maintained.
Other polymerization processes and/or downstream processing of polymers are carried out in a vehicle which is a solvent for both the monomers to be polymerized and the polymer formed. In such “solvent polymerization processes”, the separation of the polymer from the vehicle is generally an energy intensive step where the separation of the polymer from the solvent is carried out by steam stripping or other suitable solvent evaporation techniques. This is an energy intensive process. It has long been recognized that substantial economies in polymer processes could be achieved if the energy requirements of the solvent-polymer separation step could be minimized.
It is well known that many solvent-polymer solutions are stable over a limited temperature range and can be caused to separate into a solvent rich and polymer rich phase by heating or cooling. Upon heating, these solutions exhibit a lower critical solution temperature (LCST) above which separation of the polymer from the solvent system will occur. This separation results in the formation of two distinct phases, one a solvent rich phase, the other a polymer rich phase. These phase separation phenomena are generally pressure dependent, and the two phase systems can be made to revert to a homogeneous single phase by isothermally increasing the pressure of the system above a critical value which depends upon the composition of the solution and the molecular weight of the polymer. The LCST for polyisobutylene (PIB) was disclosed in Bardin, J.-M.; Patterson, D. Polymer 1969, 10, 247, the entirety of which is hereby incorporated by reference.
The LCST is that temperature above which a solution will separate into two distinct phases, a solvent rich phase and a solute rich phase. The separation phenomenon can also occur at a second lower temperature termed the Upper Critical Solution Temperature (UCST). Below the UCST a two phase separation again occurs. The measurement of LCST and UCST end points are made at the vapor pressure of the solution. The prior art teaches a number of methods of utilizing the LCST as a means for causing a polymer solution to separate into a polymer rich phase and a solvent rich phase. Illustrative prior art processes which have utilized the LCST phenomenon in polymer separation processes include those described in U.S. Pat. Nos. 3,553,156; 3,496,135; and 3,726,843. These prior art processes are disadvantageous in that a significant amount of heat energy is required to raise the temperature of the solution to affect the desired separation.
U.S. Pat. No. 4,319,021 is directed to an improvement in the foregoing phase separation processes which permits the use of lower separation temperatures. The technique described in this patent includes the addition of a low molecular weight hydrocarbon to the polymer solution. Suitable low molecular weight hydrocarbons are the C2-C4 alkenes and alkanes, which are utilized at about 2 to about 20 weight percent (wt %). While this improved process substantially reduces the phase separation temperature, heating is still required in order to affect the desired separation. Separation processes utilizing the UCST are also disadvantageous because of the need to further cool the solutions.
U.S. Pat. No. 4,946,940 is directed to a phase separation process wherein a temperature independent phase separation is reportedly caused to occur in a polymer solution by introducing into the polymer solution a critical amount of a phase separation agent. Below the critical concentration of the phase separation agent, the mixture exhibits a normal, lower critical solution temperature (“LCST”). Compounds useful as phase separation agents in the practice of this disclosure include CO2, C1-C4 alkanes, C2-C4 alkenes, C2-C4 alkynes, hydrogen, nitrogen and its various oxides, helium, neon, CO and mixtures thereof.
In such methods, a sufficient amount of a phase separation agent (PSA) is introduced into the polymer solution so that the solution, under appropriate pressures, can separate out a polymer rich phase at all temperatures between the LCST and the UCST (as determined using the pure polymer-solvent system, essentially free of the PSA.) The consequent phase separation results in a polymer rich phase and a solvent rich phase. Where methane is used as the PSA, under appropriate conditions for hydrocarbon polymers, the solvent rich phase comprises about 80% or more by volume of the total system and is substantially free of polymer.
There is need for a process technique which would allow for the economies of the afore described slurry preparation process along with the ease of separation and economies associated with phase separation of a solvent polymerization process to be carried out at or near the polymerization reaction exit temperature. In that way, little or no additional heat input would be required to affect the separation. Heretofore, such idealized, improved processes have not been achieved.
Other background references include U.S. Pat. Nos. 2,542,559; 2,940,960; 3,553,156; 3,470,143; 3,496,135; 3,726,843; 4,623,712; 4,857,633; 5,264,536; 5,624,878; and 5,527,870; U.S. patent application US2004/0119196A1; RU 2 209 213; DE 100 61 727 A; EP 014 934 2 A2; WO 02/096964; WO 02/34794; and WO 00/04061.